The telephone industry has long used lead acid batteries for back-up power to provide uninterruptible service. The typical telephone network sends signals over optical fiber from the central office to a remote terminal. There, the signals are converted from optical into electrical waves and demultiplexed onto individual copper lines bundled together as trunks for connecting to the home.
Each remote terminal supports approximately 1000 homes. The cable companies use a similar configuration, where signals are sent from the "head end" (cable company office) to remote terminals servicing approximately 500 homes. At the terminals, the signal is converted from optical to electrical waves for transmission over coaxial cable to individual subscribers. In both cases the remote terminal uses power provided by the local utility to carry the signal from the terminal to the subscriber, since fiber optic cable cannot carry electricity. To support the terminal during a utility outage, the phone or cable companies install a back-up power supply, typically an uninterruptible power supply which uses batteries as a power source.
It is desirable to eliminate batteries from these networks because of their limited life, poor reliability, and high maintenance requirements. These unfavorable attributes translate to high operating cost. Although commonly used valve-regulated lead acid batteries are referred to as "maintenance free," the batteries need continuous on-site monitoring and maintenance. The performance and life of batteries is temperature dependent. Heat degradation occurs above 77.degree. F. (for every 15.degree. F. increase above 77.degree. F. the battery life is reduced by 50%). As a result, a battery schedule for `change out` in five years only lasts two to three years. Batteries are also susceptible to "thermal runaway," which can result in the release of explosive hydrogen gas. In addition, batteries are not environmentally friendly due to lead content and are coming under increasingly strict environmental and safety regulations.
One replacement for batteries is the flywheel energy storage system. Existing systems for supporting high speed flywheels utilize either mechanical contact bearings or expensive and complicated magnetic bearing systems. Mechanical rolling element bearings have very limited life due to the high rotational speeds necessary for an effective flywheel energy storage system. Further disadvantages of mechanical bearings are noise, vibration, and poor reliability in the vacuum environment required to reduce windage losses of the high speed flywheel. A non contacting support with all control apparatus outside the vacuum solves these problems. Existing magnetic levitation systems typically are either expensive due to multiple axes of active control, or suffer from complicated magnetic structures when combining active and passive control.
U.S. Pat. No. 4,211,452 describes an inertia wheel more particularly adapted to space applications. It includes the combination of a peripheral type of motor with permanent magnet on the rotor and ironless winding on the stator. This structure limits speed due to stress. The current of the winding is switched electronically by an amplitude modulation system, associated to a reactance coefficient varying circuit, and reversal of direction of rotation of which is achieved by permutation of the control circuits. There are also provided bearings formed by a passive radial magnetic centering device and a redundant active axial magnetic centering device slaved to an axial rate detector. This device requires a permanent magnet and four control coils just for axial control.
U.S. Pat. No. 4,620,752 describes a magnetic bearing having position stabilization of a supported body which includes damping and aligning arrangement. An application of the magnetic bearing is illustrated showing a magnetic bearing system for a flywheel. This system requires combining two control coils with two rotating permanent magnets for each bearing.
It can be appreciated that new and improved magnetic levitation flywheel systems are desired, in particular, for backup power supply systems to provide uninterruptible power supplies.